Annals of Mathematics Representations of Reductive Groups Over Finite Fields Author(s): P. Deligne and G. Lusztig Reviewed work(s): Source: The Annals of Mathematics, Second Series, Vol. 103, No. 1 (Jan., 1976), pp. 103-161 Published by: Annals of Mathematics Stable URL: http://www.jstor.org/stable/1971021 . Accessed: 22/05/2012 14:56 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Annals of Mathematics is collaborating with JSTOR to digitize, preserve and extend access to The Annals of Mathematics. http://www.jstor.org Annals of Mathematics,103 (1976), 103-161 Representationsof reductivegroups over finitefields By P. DELIGNEand G. LUSZTIG Introduction Let us considera connected,reductive algebraic group G, definedover a finitefield F, with Frobenius map F. We shall be concerned with the representationtheory of the finitegroup GF, over fieldsof characteristic0. In 1968, Macdonald conjectured,on the basis of the character tables knownat the time (GLn,SpJ, that thereshould be a well definedcorrespond- ence which, to any F-stable maximal torus T of G and a character0 of TF in general position,associates an irreduciblerepresentation of GF; moreover, if T modulothe centreof G is anisotropicover F,, the correspondingrepres- entationof G' should be cuspidal (see Seminar on algebraic groups and related finitegroups, by A. Borel et al., Lecture Notes in Mathematics,131, pp. 117 and 101). In this paper we prove Macdonald's conjecture. More precisely,for T as above and 0 an arbitrarycharacter of TF we construct virtual representationsRO whichhave all the requiredproperties. These are definedin Chapter 1 as the alternatingsum of the cohomology with compactsupport of the varietyof Borel subgroupsof G whichare in a fixed relative position with their F-transform,with coefficientsin certain G F-equivariantlocally constantl-adic sheaves of rank one. This generalizes a constructionmade by Drinfeldfor SL2 (see Ch. 2). In Chapter4 we prove a character formula for RI, based on the fixed pointformula of Chapter 3. This character formula is in terms of certain "Green functions"on the unipotentelements; in Chapters 6, 7, 8 we prove that these Green functionssatisfy all the axioms predicted by Springer, Kilmoyerand Macdonald ([9], [12]). By 6.8, 4RI is irreducibleif 0 is in general positionand the vanishing theorem(9.9) gives an explicit model for it providedthat q is not too small (if G is a classical group or G, any q will do; in the general case q > 30 is sufficient). In Chapter10 we studythe irreduciblecomponents of the Gelfand-Graev representationof GF, assuming that the centreof G is connected. The proof uses the results of Chapter5 togetherwith the disjointnesstheorem (6.2). 104 P. DELIGNE AND G. LUSZTIG Finally, in Chapter11 we discuss the case of the Suzuki and Ree groups. It would be very desirable to findformulas for the Green functionsmore explicit than (4.1.2). Such formulas are known for GL, (Green [41), Sp4 (Srinivasan[13]) and G2(Chang, Ree [2]). Very recently,Kazhdan has proved, using results of Springer, that the Green functionscan be expressed as exponentialsums on the Lie algebra (see [12]) providedthat the characteristic is good and q is not too small. Some of the results in this paper were announcedin [7], [8]. The second author would like to thank the I.H.E.S. for its hospitality duringpart of the time of preparationof this paper. TABLE OF CONTENTS Conventionsand standard notations........................ 104 Chapter 1. Some basic definitions................................ 105 2. Examples in the classical groups....................... 116 3. A fixedpoint formula................................. 118 4. The characterformula ................................. 123 5. Charactersof tori..................................... 126 6. Intertwiningnumbers .................................. 135 7. Computationson semisimpleelements .................. 140 8. Induced and cuspidal representations................... 146 9. A vanishingtheorem ................................... 147 10. A decompositionof the Gelfand-Graevrepresentation .154 11. Suzuki and Ree groups................................ 160 Conventions and standard notations 0.1. In this paper, k is always an algebraically closed fieldand p is its characteristicexponent. The followingassumptions and definitionswill be in forcein Chapters 4 to 8, in Chapter 10, and in parts of Chapters 1 and 9. (0.1.1) p is a prime,and k is an algebraic closure of the primefield Fp of characteristicp. For q a power of p, Fq is the subfieldwith q elementsof k. G is a reductivealgebraic group over k, obtained by extensionof scalars from Go over Fq. We denote by F the correspondingFrobenius endomor- phism F: G G (as well as the Frobenius endomorphismof any k-scheme definedover Fq). 0.2. 1 is a primedifferent from p, and Q, is an algebraic closure of the l-adic field. When there is no ambiguity on 1, we will write simply HC(X) (resp. Hi(X)) for H,(X, Q1) (resp. Hi(X, Q1)) (l-adic cohomologyof X, a schemeover k). The groups Z/ln(1) are the groups xe1n(k)of ln-rootsof unity in k; they form a projective system, with transitionmaps x - xl'-7; the projectivelimit is Z,(1). One definesZ,(n) - Z,(1)?n, and Zl(-n) the dual of Z,(n). The symbol(n) is for a Tate twist: tensoringwith Z,(n). We will REDUCTIVE GROUPS OVER FINITE FIELDS 105 make an accidentaluse of the similarlydefined group Zp,(1)=lima , 0.3. For H a finitegroup, A(H) is the Grothendieckgroup of the finite dimensionalrepresentations of H over QC. If f and f' are class functionson H, with values in a cyclotomicfield (often c Q1), the innerproduct <f, f'>H (or simply<f, f'>) is (1/1HI) f(x)f'(x), where - is the automorphismin- ducing C --1 on the roots of unity. The same notation < , > will be used for the inner product of elementsof 9k(H) 0) Q, identifiedwith theirchar- acters. 0.4. The lengthfunction on a Coxeter group W (with canonical gener- ators sl, *. , sn) will be denotedby 1( ). A reduced expression for w e W is a decompositionw = si, s* i with 1(w) = k. 0.5. Miscellaneous: pro: ith projection(as in pr1:X x X X); XT: the fixedpoint subscheme of the endomorphismT of the schemeX (for instance: GF = Go(Fq)); p' (in index): away fromp (as in Zp,, completionaway from p) or the subgroupof elementsof order primeto p (as in (Q/Z)p); ad g: the innerautomorphism x gxg', or maps deducedfrom it; 1, or e: the identityelement of a group; Tr(f, V*), forf an endomorphismof a graded vector space, is E (- 1)' Tr (fyVi) ; reductive: reductivegroups are meant to be connectedand smooth. The Jordandecomposition of an elementg C G (G as in (0.1.1)) is g = su where s is a semisimpleelement and u is a unipotentelement commuting with s. 0.6. We will oftenidentify a schemeover k withthe set of its k-rational points. This should cause no confusion. 1. Some basic definitions 1.1. Suppose that in some category we are given a family (Xi)iI of objects and a compatiblesystem of isomorphismsTj: Xi > Xi. This is as good as giving a singleobject X, the "commonvalue" or "projectivelimit" of thefamily. This projectivelimit is providedwith isomorphisms vi: X Xi such that j'iu = qaj. We will use such a construction to define the maximal torus T and the Weyl group W of a connectedreductive algebraic group G over k. As index set I, we take the set of pairs (T, B) consistingof a maximal torus T and a Borel subgroup B containingT. For i C I, i = (Ty B), we take 106 P. DELIGNE AND G. LUSZTIG Ti = T, W. = N(T)/ T. The isomorphism'i3 is the isomorphisminduced by adg whereg is any elementof G conjugatingi intoj; these elementsg form a single right Ti-coset,so that (pi is independentof the choice of g. One similarlydefines the root system of T, its set of simple roots, the action of W on T and the fundamentalreflections in W. Let F: G-Gbe an isogeny. For any i CI, i=(T.B),F(i)=(F(T),F(B)) is again in I; F inducesan isogenyF: Ti) TF(i) and an isomorphismF: Wi WF(i). The correspondingendomorphism (resp. automorphism)a-'i)Fai of thetorus T (resp. of the Weyl groups W) is independentof the choiceof i; we say that it is induced by F. 1.2. Let X (or XG) be the set of all Borel subgroupsof G. The group G acts on X by conjugation,and X is a smoothprojective homogeneousspace of G. For each Borel subgroup B of G, B is the stabilizerof the correspond- ing pointof X, hence thereis a natural isomorphismGIB - X: g v--gBg-'. The set of orbitsof G in X x X can be identifiedwith the Weyl group W of G as follows:for any i (T, B) as in (1.1), use the compositebijection W N( T)IT B\GIB G\(GIB x GIB) G\X x X; Ui ~~~~~~~~~(e,g) this is independentof the choice of (T, B). We denote by O(w) the orbit correspondingto we W; this is the orbit of (B, &Bil-'), where lb e N(T) representsw. We shall say that two Borel subgroups B', B" of G are in relative position w, w C WYif and only if (B', B") C O(w). In diagrams, we will picture this by B' - B". The basic properties of Bruhat decompositioncan be expressed as follows: (a) If w = w1w2,with l(w) = 1(w1)+ 1(w2)then (a,) (B', B") c O(w1)and (B", B"') e O(w2) (B', B"') e O(w); (a2) if (B', B"') e O(w), there is one and only one B" such that (B', B") c O(wl) and (B", B"') G 0(w2).